a) Field of the Invention
The present invention relates to optical recording media which permit writing once with a convergent beam of a laser, especially a semiconductor laser, and more specifically to recording media useful for external memories of computers and for recording various information such as video and audio information. The present invention is also concerned with a process for the production of optical recording media.
b) Description of the Related Art
Recording media bearing an inorganic recording layer formed of a thin film of a low-melting-point metal such as tellurium, a tellurium alloy or a bismuth alloy have heretofore been proposed as write-once optical recording media. Some of these conventional recording media have already been used actually.
In recording media having a thin layer of such a low-melting-point metal as a recording layer, the formation of the recording layer is conducted by a vacuum process such as vacuum evaporation, sputtering or ion plating. These film-forming processes however do not permit easy control of the thickness, composition, etc. of a recording layer, so that they involve problems in productivity and yield. Moreover, recording media having as a recording layer a thin film of such a low-melting-point metal are accompanied by another drawback that thermal energy produced by a laser beam irradiated upon recording of signals is caused to dissipate through the thin metal film because the thermal conductivity of the recording layer is high. As a result, irradiated energy cannot be used effectively for the recording of signals, resulting in reduced recording sensitivity, excessively large record pits and irregular pit size. There is hence a limit to the recording density. Further, metals used in recording layers, such as tellurium, are prone to oxidation and are not fully reliable in durability and moreover, have toxicity and are dangerous.
Optical recording media making use of an organic dye as a recording layer have also been proposed. With these recording media, improved productivity, recording sensitivity and recording density are feasible since an organic dye can be easily formed into a film by a coating method such as spin coating and moreover generally has a lower thermal conductivity compared to metals.
Recording media which contain as a recording layer an organic dye having an absorption in a semiconductor laser range have also been developed. Examples of such an organic dye include dithiol metal complexes, polymethine dyes, squarylium dyes, naphthoquinone dyes, phthalocyanine dyes, and naphthalocyanine dyes. Some of these recording media have found practical utility.
The optical recording media proposed to date, which contain an organic dye as a recording layer, are however accompanied by the drawbacks that blisters may occur at edges of pits upon recording (rim formation) or the quality (CN ratio) of recorded signals may be lowered due to the formation of decomposition products and they may not be suited for applications where a high CN ratio is required, for example, for analog signals. They may not be able to give large reading-out power because the threshold upon recording is not sharp.
Japanese Patent Laid-Open No. 164037/1988 discloses a recording medium capable of suppressing the formation of rims and/or the occurrence of decomposition products, in which an overcoat layer composed of an organopolysiloxane formed by the condensation of phenyltriethoxysilane and methyltriethoxysilane is provided on an organic dye layer and recording is performed by the formation of bubbles. As will be demonstrated subsequently in Comparative Example 1, the noise-to-signal ratio (CN ratio) of this recording medium tends to decrease when recorded under an excessive laser power (i.e., over-power), so that it is impossible to set large the margin of the recording laser power. When recording is performed in a mode of a constant angular velocity, the linear velocity varies depending on the radial distance from the center of the recording medium so that the recording laser power must be precisely controlled to always maintain it at an optimal level. The CN ratio is lowered by variations in laser power upon recording. Further, when recording is performed by the mark-length recording method which is a high-density recording method, good CN ratios can be obtained when pit lengths are short but cannot be achieved when pit lengths are long.